(1) Technical Field
The present invention relates to the field of radio-frequency (RF) waveguides. More specifically, the present invention pertains to a method and apparatus that provides ultra-low-loss (less than 1 dB per meter) RF waveguide structures targeted between 300 GHz and 30 THz.
(2) Background
RF waveguides are based on the theory of wave propagation. The theory of wave propagation in periodic structures was developed several years ago. That theory provided fundamental understanding in solid-state physics and x-ray diffraction. Since then, the theory has been expanded for many other applications, such as traveling wave tubes, bandpass filters, distributed feedback lasers, integrated optics structures, dichroic plates, artificial dielectrics, etc. Most recently, photonic periodic structures (or photonic crystals) using the same basic theory have been developed for a wide range of traveling wave applications in both the microwave and optical regions of the electromagnetic spectrum. The periodic nature of the photonic crystal elements allows propagation over a restricted band of wavelengths, thus allowing filters, couplers, power splitters, waveguides and many other components, to be realized. The constructions that give rise to this frequency-dependent behavior are commonly termed “photonic bandgap structures” (PBG structures).
Hollow core photonic bandgap fiber has been developed in large part by P. J. Russell, J. C. Knight and colleagues at the University of Bath (located at BA2 7AY, United Kingdom) over the past 10 years, as disclosed in the following papers:
(1) Philip Russell, “Photonic Crystal Fibers,” Science, v.299, pp. 358-362, Jan. 17, 2003.
(2) G. Humbert, J. C. Knight, G. Bouwmans, P. J. Russell, D. P. Williams, P. J. Roberts and B. J. Mangan, “Hollow core photonic crystal fibers for beam delivery,” Optics Express, Vol. 12, no. 8, pp. 1477-1484, 19 Apr., 2004.
(3) B. J. Mangan, L. Farr, A. Langford, P. J. Roberts, D. P. Williams, F. Couny, M. Lawman, M. Mason, S. Coupland, R. Flea, H. Sabert, T. A. Birks, J. C. Knight and P. J. Russell, “Low loss (1.7 dB/km) hollow core photonic bandgap fiber,” Optical and Fiber Comm. Conf (OFC-2004) post deadline paper PDP24, Los Angeles, Calif., Feb. 22-27, 2004.
(4) T. A. Birks, D. Mogilevtsev, J. C. Knight and P. J. Russell, “Dispersion compensation using single-material fibers,” IEEE Photonics Technology Letters, vol. 11, no. 6, pp. 674-676, June 1999.
(5) J. C. Knight, T. A. Birks, P. J. Russell and D. M. Atkin, “All-silica single-mode optical fiber with photonic crystal cladding,” Optics Letters, vol. 21, no. 19, pp. 1547-1549, Oct. 1, 1996.
These investigators have targeted the optical fiber community, especially the near infrared, and optimized hollow core (HC)-PBG structures for both low-loss and high power continuous wave (CW) and pulsed propagation between 1 and 2 microns, with potential extension to 10 microns wavelength.
The article by Knight and Russell et al. discloses a hollow core photonic bandgap fiber developed for 1.5 micron propagation. The core is 20 microns in diameter and the honeycomb is approximately 4 microns. The material employed is silica and the structures are extruded through a mandrel in a hot-melt process, allowing for the fabrication of many geometries.
Knight and Russell et al. developed another optical HC-PBG design, which is disclosed in G. Humbert, J. C. Knight, G. Bouwmans, P. J. Russell, D. P. Williams, P. J. Roberts and B. J. Mangan, “Hollow core photonic crystal fibers for beam delivery,” Optics Express, Vol. 12, no. 8, pp. 1477-1484, A9 April, 2004. This second design disclosed a reduced diameter core and substantially higher propagation loss. Knight and Russell et al. focused on 7-cell and 19-cell core designs.
The idea of using photonic bandgap structures for terahertz waveguide propagation was published by H. Han, H. Park, M. Cho and J. Kim, in “Terahertz pulse propagation in plastic photonic crystal fibers,” Lasers and Electro-Optics, 2001 and CLEO/Pacific Rim 2001, Volume: Supplement, paper WIPD1-10, pp. 22-23, 19 Jul. 2001, and later embellished in H. Han, H. Park, M. Cho, J. Kim, I. Park and H. Lim, “Terahertz pulse propagation in plastic photonic crystal fibers,” 2002 IEEE MTT-S Int. Mic. Sym. Digest, vol. 2, pp. 1075-1078, Jun. 7, 2002.
Hans et al. disclosed a plastic PBG guide with a solid polyethylene core and surrounding an equal-sized honeycomb sheath for terahertz pulse propagation. However, the measured loss is high, greater than 5 decibels per centimeter (dB/cm) at 400 gigahertz (GHz).
The above-mentioned PBG guides are typically used for fiber optic communications. However, such PBG guides are not suitable for use in terahertz endoscopes and fiberscopes. Accordingly, a need exists for an apparatus and method that provides a low-loss radio-frequency (RF) waveguide structures between sub-millimeter (300 GHz) and far-infrared (30 THz).